In the past, the paper-making industry has used substantial quantities of wood chips which were produced from debarked logs. In 1970, a new chipping apparatus was first introduced, see U.S. Pat. No. 3,661,333, which was capable of reducing an entire tree, with attached limbs and branches, to chips. Although originally designed to serve as a tree-destroying apparatus to simplify removal of trees as chips, rather than bulley limbs and branches, as in land clearing operations or removal of diseased or dead trees from city streets, it was immediately recognized that in performing its function the machine of U.S. Pat. No. 3,661,333 produced large quantities of wood chips which would be useful in the papermaking industry. However, the paper-making industry requires chips of a substantially uniform size and the chips produced by the machine of U.S. Pat. No. 3,661,333 because of the relatively thin branches and twigs present on the undelimbed trees which it handled, produced chips which included chips of widely varying sizes and dimensions.
The prior practice of the paper-making industry in employing debarked logs as the material from which chips were produced enabled the production of chips of the desired uniform size in that the chipper always operated on material of uniform cross-section from which the bark had previously been removed. However, this process required the removal of all limbs from the felled tree and the subsequent removal of the bark from the delimbed log. The delimbing of the felled tree and the subsequent debarking of the log were relatively costly steps in terms of the labor and equipment involved; further, many of the delimbed and debarked logs so produced represented merchantable timber which would have substantially greater value as lumber rather than paper pulp. The machine of U.S. Pat. No. 3,661,333 on the other hand, largely handled trees having little or no economic value as lumber, however, the chips produced by such machine found those chips usable by the paper-making industry intermixed with substantial quantities of over or under sized chips, bark, and finely shredded leaves and small twigs.
While this mixture of usuable and unusable chips immediately suggested a screening or separating operation to cull the good chips from the bad, some difficulty was encountered in achieving an efficient chip separating operation. In order to be feasible, the screening operation had to be capable of handling large quantities of wood chips which meant that relatively large quantities of chips were dumped upon the first or coarse separating screen. With a layer of chips several layers deep on the coarse separating screen, the larger chips tended to exert a clogging effect on the coarse screen and unless this first stage of separation was a very thorough one, substantial quantities of usable chips were carried off the coarse separating screen by the larger chips. Efforts to resolve this problem usually involved the employment of relatively large area coarse screens combined with a relatively slow transit time of wood chips across the screen, both of these characteristics representing an economic penalty.
The present invention is directed to this latter problem and provides a vibratory screen apparatus of relatively simple and compact construction which efficiently separated chips of a usable size from other chips of larger and smaller sizes.